The invention relates to a vane pump which may find its application in power steering apparatus for vehicle or the like.
Generally, a vane pump includes a cam ring having a substantially elliptical cam surface formed around its inner periphery, a rotor disposed for rotation within the cam ring, a plurality of vanes which are slidably disposed in slits formed around the outer periphery of the rotor at an equal circumferential interval, and plates which hold the combination of the cam ring and the rotor sandwiched therebetween. As the rotor rotates, each vane moves in sliding contact with the cam ring to increase or decrease the volume of the pump chamber which is defined between a pair of mutually adjacent vanes, thus performing an oil suction or discharge operation.
In a vane pump as described, the oil which is discharged from the pump is introduced into the inside of each slit in order to drive each vane through the slit of the rotor to achieve a positive contact of the vane with the internal surface of the cam ring. In the conventional practice, in their surfaces which are adapted to abut against the rotor, both of the plates are formed with four arcuate grooves which are positioned on a common circle, with the discharged oil being directly introduced into two of these grooves while the discharged oil is introduced into the remaining two grooves through a narrow path which communicates with the grooves.
Upon starting the vane pump, the vane which is slightly driven outward by the centrifugal effect, will abut against the cam to be pushed back in the opposite direction upon entering discharge stroke, whereby the oil is expelled from the slit into which the vane is driven. The expelled oil passes through the narrow path to be delivered to the discharge side of the pump. A flow resistance which the oil experiences during its passage through the narrow path causes an increase in the oil pressure inside the slit, thereby urging adjacent vanes outward which have not yet been significantly driven outward. By repeating such process, all of the vanes will eventually be completely driven outward for abutment against the cam ring to initiate the individual discharge operation.
The conventional vane pump as described above suffers from a number of difficulties, including poor starting response which is caused by a failure of a sufficient rise in the oil pressure inside the vane during its low speed rotation if the path which interconnects the arcuate grooves formed in both of the plates is not narrow enough, and an increase in the power loss and a rise in the oil temperature which may be caused by an increased flow resistance occurring during a high speed operation if the path is made excessively narrow.